Add capability of adding and starting new sites at runtime

Allow users to create and modify site config at runtime through a public API and eventually in Beacon Live Admin. Currently the config is passed as options to the `SiteSupervisor` but we'll need to store such config in the database.

Start by tracing how options are passed to SiteSupervisor and identify the existing runtime site lifecycle. Clarify the public API and database persistence scope before implementation; done means users can create and modify site configuration at runtime and start new sites, with Beacon Live Admin support treated as a later goal.

Written by the indexing model from the issue text.